The present invention relates to a method of removing sulphur compounds from a gas stream. In this method, the sulphur compounds are first converted into hydrogen suilphide, and the hydrogen sulphide is then, in an aqueous solution, converted into elemental sulphur by biological oxidation.
As far as the oil and gas(-processing) industry is concerned, sulphur compounds are among the most important pollutants which can occur in off-gases. It is a pollutant which occurs at high concentrations, and legislation relating to these compounds is very strict.
Consequently, many processes are known for removing sulphur compounds from gasses. One of the most important processes comprises the catalytic conversion of the sulphur compounds to elemental sulphur. A major advantage of this process is that the elemental sulphur is a product of intrinsic economic value.
The most important process for converting sulphur compounds, especially hydrogen sulphide, into elemental sulphur is the so-called Claus process. Using this process, a total sulphur removal of about 95% can be achieved. The residual quantity of sulphur is contained in the so-called xe2x80x9cClaus off-gasxe2x80x9d (sometimes referred to as xe2x80x9cClaus tail gasxe2x80x9d) in the form of sulphur compounds such as COS, CS2, SO2 and SO3, but also small amounts of gaseous elemental sulphur (Sx) and mercaptans (RSH).
In other processes too, sulphur-containing gas streams can be formed, such as synthesis gas or fuel gas, which still contain the abovementioned undesirable sulphur compounds. These sulphur compounds are often converted into hydrogen sulphide by means of catalytic hydrogenation or catalytic hydrolysis.
An example of the catalytic hydrogenation of these compounds to produce hydrogen sulphide is the so-called SCOT process (Shell Claus Off-gas Treatment). This process converts the sulphur compounds present in the off-gas of the Claus process. Usually, the hydrogen sulphide thus formed, having been selectively removed by means of an amine-containing solution, is recycled to the Claus reactor to increase the efficiency of the latter.
Even the off-gas of said SCOT process may still contain traces of sulphur components, generally traces of COS and CS2. To prevent emission of these compounds, this process will generally comprise an afterbuming step, in which these compounds are converted into SO2.
Alternatively, the undesirable sulphur compounds can be converted to hydrogen sulphide by catalytic hydrolysis. Such a process is used, in particular, in gas streams which contain carbonyl sulphide (COS). Characteristic gas phase hydrolysis catalysts are based on copper sulphide, chromium oxide, chromium oxide/aluminium oxide, and platinum.
Another example of a process in which sulphur compounds present in the Claus off-gas are converted to hydrogen sulphide is the so-called Beavon process. This process is used to remove sulphur compounds from off-gas of the Claus process by means of hydrolysis and hydrogenation over a cobalt molybdate catalyst, which results in the conversion of carbonyl sulphide, carbon disulphide and other sulphur compounds into hydrogen sulphide.
It is an object of the present invention to provide a method of converting hydrogen sulphide into elemental sulphur, said method making use of aerobic bacteria. Consequently, the resultant H2S from the catalytic reduction step will not be recycled to the Claus reactor. This application is of interest, in particular, if a Claus installation of this type is unavailable, as is the case, for example, with so-called stand-alone SCOT units.
Another object of the present invention is to carry out this conversion in such a way as to thereby achieve high efficiency.
A method of removing hydrogen sulphide from a gas stream has now been found wherein the hydrogen sulphide is scrubbed from the gas phase by means of an aqueous solution, the hydrogen sulphide in the aqueous solution is biologically oxidized in a bioreactor to produce elemental sulphur, and the elemental sulphur is separated from the aqueous solution, characterized in that the gas stream to be treated is cooled to such a degree that at least sufficient water vapour condenses from said gas stream to compensate for the discharge stream for the purpose of removing salts. This means that no water need be supplied to the bioreactor, and it may even be possible to produce good-quality water.
The method according to the present invention further provides the following advantages:
Using this method, it is possible for elemental sulphur to be obtained with a high yield from gas streams which contain hydrogen sulphide.
Any HCN present in the gas stream reacts with elemental sulphur to form thiocyanate (SCNxe2x88x92) which is biodegraded.
Even traces of other sulphur compounds still present in the gas stream are converted.
The process has low energy consumption.
No expensive chemicals are required.
The process can be operated in a simple manner.